Synthetic polyester yarns have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251, and then by Whinfield and Dickson, U.S. Pat. No. 2,465,319. In particular, polyester staple fiber has been an industrial commodity that has been manufactured and used in spun textile yarns on a very large scale, primarily in blends with natural fibers, especially cotton, such blends having been spun (twisted) into spun yarns that have been made into textile fabrics, and eventually into garments and other textiles. A typical spun textile yarn is of cotton count 25, containing a cross-section of about 140 fibers of 11/2 denier and 11/2 inches cut length, for example, but the denier and cut length can vary up to about 3 and down to about 1. Because of the sophistication of the textile industry, both of the polyester fiber manufacturing industry and of downstream consumers of textiles, and because of the commercial interest in providing apparel and fabrics that will perform well during actual use by the ultimate consumer (wearer), much attention has been devoted to analyzing appropriate requirements. Many technical papers, for example, have been published on various aspects, and patents have been issued with the objective of improving the "comfort" that can be obtained from textile articles, and their constituents, and the literature has been replete with these suggestions for several years. So it has long been considered desirable to improve the comfort properties obtainable from textiles prepared from spun textile yarns of polyester staple fiber, and much effort has been devoted in the textile industry towards this objective.
There has also been increasing interest in providing polyester fiber of low shrinkage for making spun yarns. Low shrinkage means low and uniform shrinkage, especially to avoid high shrinkage tensions (such as occur usually with polyester fiber that has not been processed to reduce its shrinkage), to avoid problems subsequently, often referred to as downstream, during processing of the yarns or fabrics. Mixed shrinkage is not desired according to the present invention. Apart from other characteristics, the shrinkage of higher shrinkage components causes a tightening up that is not desired in yarns according to the present invention. The present application, therefore, concerns polyester fiber that is of desirably low and uniform boil-off shrinkage, preferably less than about 3%, and especially less than about 1%. The dry heat shrinkage, measured at 196.degree. C., is preferably less than about 6%.
An important objective of my invention is to provide such polyester staple fiber in a new form and to process it into spun yarns, which can then be formed into fabrics and garments that can show improved comfort properties, as discussed hereinafter.
Polyester staple fiber of low shrinkage has generally been manufactured commercially by a process of melt spinning (i.e. extruding molten polyester polymer) into a bundle of filaments, collecting such filaments into a tow, which can be relatively small and converted directly, e.g. by stretch-breaking, into a spun yarn, but has more often been extremely large, amounting to many thousand and even some million(s) of filaments, and this tow has then been processed by drawing, treating to reduce shrinkage, and crimping, and the crimped low shrinkage filaments have been converted into staple fiber by cutting, or otherwise, to the desired lengths. As indicated, polyester staple fiber has often then been blended, e.g. with cotton, and converted into yarn, which is generally referred to as a spun yarn, to distinguish it from a continuous filament yarn. The natural fibers, such as cotton, with which the polyester staple has often been blended have not been uniform. For instance, they vary in size, shape and surface properties to some extent. The natural characteristics of cotton have long been believed to be responsible for the attractive qualities of the spun yarns, and of the articles, such as fabrics and garments, prepared therefrom, and much effort has been devoted to duplicating various characteristics of cotton. Nevertheless, so far as I know, polyester staple has been sold commercially as of uniform nominal denier (denier being the weight in grams of 9000 meters of a staple fiber, continuous filament or yarn, and thus being a measure in effect of the thickness of the fiber, filament or yarn; in fact, since staple fiber is, by definition, of short cut length, about 1 to 3 inches, the denier must be calculated by extrapolation or must be measured on the precursor tow or, more precisely, on random extracts of a specified number of continuous filaments from the tow). When one refers to uniform denier, the nominal denier, i.e. average denier, is referred to, since there is inevitable variation along-end and end-to-end. However for commodity fibers, as opposed to some specialty fibers, it has generally been the objective of fiber producers to achieve as much uniformity as possible when melt-spinning, drawing and reducing shrinkage and thus to minimize variations between individual filaments (i.e. end-to-end) and along the individual filaments (i.e. along-end), so as to produce a polyester fiber product of as uniform denier as practical. This is the present commercial practice. Polyester fiber producers sell tow or staple fiber of various nominal deniers. It would have been possible for anyone to buy polyester staple fiber (or tow) of various different deniers, and to blend them together, if desired with natural fibers, such as cotton. I do not know that anyone has actually done this, but it would have been quite possible. I believe that polyester staple fiber or tow of intentionally mixed denier has not previously been sold as an article of commerce. Polyester fiber is usually sold compressed into bales. I believe bales of polyester fiber of intentionally mixed denier have not previously been sold as articles of commerce.
The present invention is not concerned with continuous filament yarns, but with staple fiber for making spun yarns, which have entirely different aesthetics and are prepared by different techniques.
Wada et al., U.K. Patent Application GB 2 039 560A (Wada), concerns a multi-layered bulky spun yarn comprising at least three kinds of staple fibers which vary in denier. Wada mixes together fibers of different thermal shrinkage into a sliver which is wrapped around a roving of fibers of a third kind, to get a double layered roving, which is then spun into a fine spun yarn, which is subjected to heat treatment. This heat treatment causes layering of the different fibers, because of their different shrinkages, so that the heat-treated yarn has an outer layer of fiber of fine denier and a core of fibers of high denier, separated by an intermediate layer of fibers of intermediate denier, as shown in FIG. 1B of Wada, as contrasted with FIG. 1A, before heat treatment. As has been explained, mixed shrinkage is not desirable according to the present invention (nor is Wada's layering, as will be apparent).
Sekiguchi, U.S. Pat. No. 3,604,197 teaches multicolored yarns made from a blend of different denier fibers, preferably of differing shrinkages, so the higher deniers collect in the core, and the finer deniers emerge at the yarn surface. The present invention is not concerned with such multicolored yarns, but with polyester fibers of uniformly the same color or substantially the same color, as indicated herein.